Sunday, July 12, 2015

Reverse total shoulder arthroplasty- glenosphere dissociation and how to avoid it.

Glenosphere dissociation after reverse shoulder arthroplasty.

These authors reviewed their reverse total shoulder arthroplasty database and identified 13 patients with glenosphere dissociation between 1999 and 2013; dissociation occurred 0.5 months to 7 years postoperatively.

Incidence of dissociation was correlated to glenosphere size (p < .001). Dissociated glenosphere size distribution was as follows: 32 mm (n = 1), 36 mm (n = 4), 40 mm (n = 6), and 44 mm (n = 2).

The authors noted that improper taper engagement reduced the torsional capacity of the glenosphere-baseplate interface.

Among retrieved glenospheres, the authors found limited evidence of fretting wear between the glenosphere and baseplate.

Comment: This paper re-emphasizes the risk of glenosphere dissociation that we previously posted on here and here and here - please visit these posts for additional details. We observe that to achieve a secure Morse taper lock between the glenosphere and the base plate, there must be no soft tissue or bone that prevents full seating when the glenosphere is impacted into position. Even the slightest blockage to full seating will virtually eliminate the security of the Morse taper lock. Tightening the 'set screws' that are a part of many systems cannot offset an incompletely seated Morse taper.

As the authors point out, glenospheres with large diameters provide greater opportunity for interference with complete seating. In some systems, the instrumentation removes bone for the small but not the larger sizes as shown below.

Thus if a larger glenophere is used, the surgeon must carefully remove potentially blocking bone from around the perimeter of the base plate.  The principal method by which the seating can be verified is to pull vigorously on the glenosphere after it has been impacted into position, attempting to dissociate it from the baseplate. With some designs, vigorous traction can be applied using a t-handled instrument. An even better test can be performed by attempting to twist the glenosphere using the t-handle: if it twists on the baseplate, it is not securely seated.

After surgery, patients need to be reminded that impact loading is to be avoided.

Our current reverse total shoulder technique is shown in this link.


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